Now, that's a question that can have a myriad of answers.... excitement, curiosity, ego-boosting, sexual boredom (same ol'/same ol'), feeling as though they are not understood by their primary partner.
Infidelity causes extreme stress, upheaval and turmoil to a relationship, often times in a way that cannot be reconciled. Why do it? The risk is high and the consequences are severe. If there are children, they are greatly and severely affected, as well.
The truth is that it's all a myth. We, humans, aren't designed to engage in affairs of lust & passion. It's unrealistic, and it just doesn't work for the long-haul.
Listen, you guys and some of you women ~ that someone who is there for you, who cares for you, for the long-haul is the person who needs to be nurtured, supported, loved and validated. Real happiness comes from that. And it takes commitment. AND, that is the REAL DEAL.